In most known date display devices, the positioning of the date ring in the successive display positions is generally ensured by a jumper spring associated with the date ring toothing. Conventional drive systems do not ensure sufficient retention of the date ring in the event of shocks and therefore generally any locking. Thus, it is the jumper spring that has to ensure this locking function, which is why it has a high elastic constant. It will be noted that, to overcome the elastic force of the jumper spring, a high torque must be provided at the date ring.
However, to overcome the problem of the positioning jumper spring also having an anti-shock function, EP Patent Application 2927756 proposed a date ring drive mechanism including an irreversible transmission system capable of ensuring the anti-shock function while the positioning of the date ring is still ensured by a jumper spring, which then has a lower elastic constant. This document proposes, in particular, one embodiment with a pinion formed of two diametrically opposite pins relative to the axis of rotation of the pinion. In this document, a large play is provided between the pin and the ring toothing to ensure interference-free driving, particularly without locking. As the positioning jumper spring holds or returns the ring in or to a position where the axis of rotation of the pinion substantially intercepts the central axis of the tooth inserted between the pins, the pins penetrate the hollows of the toothing without risk of locking. To ensure this function, the pins are even truncated on the side of the rotational axis. It will be noted that the toothing has hollows with sides that close towards the bottom of the toothing. One problem with the embodiment represented in FIGS. 3A and 3B of EP Patent Application 2927756 arises from the fact that the pins have a small diameter and are also truncated, so that they are fragile and risk being damaged, and especially bent in the event of shocks. Further, this embodiment requires a positioning jumper spring, which increases the size of the date display device and also the cost of the timepiece movement.
Moreover, there is known from U.S. Pat. No. 6,185,158 an electronic watch fitted with an analogue display for several time parameters, in particular the hours, minutes and seconds, by means of three coaxial hands located at the centre of the watch dial. Further, the analogue display includes a chronograph hand, in particular a minute hand for the measured time interval, associated with a circular graduation over 360°, and a date display utilising a date ring, the displayed date appearing in conventional manner through an aperture in the dial. This Patent proposes to actuate the mechanism driving the chronograph hand (hereafter the “first mechanism”) and the mechanism driving the date ring (hereafter the “second mechanism”) via the same single electromechanical motor.
The first mechanism includes an intermediate wheel driven directly by the rotor of the motor and a chronograph wheel that meshes with the intermediate wheel. The second mechanism also includes said intermediate wheel and also an auxiliary wheel meshing with said intermediate wheel. The auxiliary wheel is integral with a wheel set that periodically actuates a wheel driving the date ring, this wheel set having a finger for actuating the drive wheel. The periodic actuation wheel set and the drive wheel together form a Geneva mechanism, known for periodically driving a date ring/disc. On each rotation of the periodic actuation wheel set, the finger drives the date ring drive wheel, which is driven in rotation over an angular distance corresponding to the change from one date to the next in the aperture in the dial provided for the date display. The Geneva mechanism is thus characterized by the periodic driving of the date ring drive wheel, with the periodic actuation wheel set only meshing with the drive wheel over an angular sector of less than 360°, whereas the wheel set locks the drive wheel on the remaining angular sector. Thus, although the periodic actuation wheel set rotates when positioned in the remaining angular sector, the rotational motion of the rotor is not transmitted to the date ring.
U.S. Pat. No. 6,185,158 uses the Geneva mechanism to enable the motor used for driving the date mechanism to perform an additional function, namely driving a chronograph hand. In short, the method consists in driving the chronograph hand when said periodic actuation wheel set is in its area of non-actuation, i.e. in said remaining angular sector, and, at the end of the measured time interval, in performing a reverse reset to return the periodic actuation wheel set to a predefined initial position.
The Geneva mechanism or a similar Maltese cross mechanism are relatively complex for the driving of a date ring. They require low tolerances for such a mechanism to be efficient and there is a risk of locking. Moreover, they are relatively bulky.